SEOUL, South Korea — South Korea on Friday urged North Korea to immediately release two of its citizens detained in the country over alleged espionage, the latest in a series of arrests in the North of foreign nationals.

The North's state media said late Thursday that the two were detained last year for allegedly collecting confidential state information and attempting to spread "bourgeois lifestyle and culture" in the North at the order of South Korea's spy agency and the U.S. It identified the men as Kim Kuk Gi and Choe Chun Gil and said the two acknowledged their acts during what was described as a news conference in Pyongyang.

North Korea has occasionally detained South Koreans, Americans and other foreigners on accusations of spying in what analysts say are attempts to wrest outside concessions. Authorities in Pyongyang in the past staged news conferences, during which foreign detainees appeared before the media and made statements that they recant after their releases.

On Friday, South Korea's Unification Ministry confirmed that Kim and Choe were South Korean citizens but denied they were engaged in espionage operations. Ministry officials could not explain how the two ended up in the North.

South Korea's National Intelligence Service, the country's main spy agency, denied the North's accusations of spying.

The North's Korean Central News Agency said Kim was detained in September in Pyongyang and Choe near the border with China in December.

According to KCNA, Kim said Thursday that he had gathered information about a railway station that late North Korean leader Kim Jong Il passed during his visit to China in 2009 and photos of its vicinity, and had spread comic books critical of the North's leadership, while Choe said he had smuggled CDs and USB memory sticks containing sex films and South Korean movies into the North.

North Korea has been holding another South Korean man since late 2013 for allegedly spying and trying to set up underground churches in the North. The country last year sentenced him to hard labor for life.

Last year, the North released three Americans — two who had entered the country on tourist visas and Kenneth Bae, a Korean-American missionary who had been convicted of "anti-state" crimes. An Australian missionary detained for spreading Christianity was also deported last year after he apologized for anti-state religious acts and requested forgiveness.

The latest incident comes amid ongoing regular military drills between South Korea and the U.S., which Pyongyang calls a rehearsal for an invasion. South Korean and U.S. officials say the drills are defensive in nature.

Thursday also marked the fifth anniversary of a sinking of a South Korean warship that Seoul officials blame on a torpedo attack by the North. Pyongyang has denied any involvement in the sinking that killed 46 South Korean sailors.